DESCRIPTION

KTR provides a circular buffer of events that can be logged in a
printf(9) style fashion. These events can then be dumped with ddb(4),
gdb(1) or ktrdump(8).
Events are created and logged in the kernel via the CTRx macros. The
first parameter is a mask of event types (KTR_*) defined in The event
will be logged only if any of the event types specified in mask are
enabled in the global event mask stored in ktr_mask. The format argument
is a printf(9) style format string used to build the text of the event
log message. Following the format string are zero to five arguments
referenced by format. Each event is logged with a file name and source
line number of the originating CTR call, and a timestamp in addition to
the log message.
The event is stored in the circular buffer with supplied arguments as is,
and formatting is done at the dump time. Do not use pointers to the
objects with limited lifetime, for instance, strings, because the pointer
may become invalid when buffer is printed.
Note that the different macros differ only in the number of arguments
each one takes, as indicated by its name.
The ktr_entries variable contains the number of entries in the ktr_buf
array. These variables are mostly useful for post-mortem crash dump
tools to locate the base of the circular trace buffer and its length.
The ktr_mask variable contains the run time mask of events to log.
The CPU event mask is stored in the ktr_cpumask variable.
The ktr_verbose variable stores the verbose flag that controls whether
events are logged to the console in addition to the event buffer.

SEEALSO

HISTORY

The KTR kernel tracing facility first appeared in BSD/OS 3.0 and was
imported into FreeBSD 5.0.

BUGS

Currently there is one global buffer shared among all CPUs. It might be
profitable at some point in time to use per-CPU buffers instead so that
if one CPU halts or starts spinning, then the log messages it emitted
just prior to halting or spinning will not be drowned out by events from
the other CPUs.
The arguments given in CTRx() macros are stored as u_long, so do not pass
arguments larger than size of an u_long type. For example passing 64bit
arguments on 32bit architectures will give incorrect results.